A trocar canula which is disclosed in DE-B-3011069, comprises at least two opposed slotted clamps for securing fascial holding sutures, and a freely longitudinally displaceable tapered seal. This trocar canula has the disadvantage that, after its insertion into an opening in the abdominal wall, the trocar canula and the tapered seal assume an unalterable position relative to one another since the said sutures which are firmly connected to the trocar canula determine its position and that of the tapered seal, in the opening in the abdominal wall. The relative positioning of the tapered seal and trocar canula cannot be altered without disturbing their positioning in the opening in the abdominal wall, thereby destroying the gastight closure of the abdominal cavity.